Microstructure and Properties of M₃B₂-Type Boride-Based Cermet Coatings Prepared by Laser Cladding Synthesis

• Main Authors: Zhaowei Hu, Wenge Li, Yuantao Zhao
• Format: Article
• Language: English
• Published: MDPI AG 2019-07-01
• Series: Coatings
• Subjects: boride-based cermet; laser cladding synthesis; hardness; corrosion resistance
• Online Access: https://www.mdpi.com/2079-6412/9/8/476

Although Q235 steel materials are widely used in offshore engineering, the service life is severely shortened by its inferior resistance to wear and corrosion in harsh marine working environments. Boride-based cermet composites could be a good surface-protective coating to enhance surface hardness, wear resistance, and corrosion resistance. M₃B₂ (M: Mo, Ni, Fe, Cr) boride-based cermet coatings composed of hard ceramics of M₃B₂-type complex borides and an {Fe, Ni} metal matrix was fabricated on Q235 steels with mixed Mo, Cr, B, and Ni60 powders using a laser cladding synthesis technique. The influences of laser cladding parameters on the microstructure, phase composition, microhardness, and corrosion resistance of the coatings were comprehensively investigated. Results showed that the microstructures of the coatings mainly consisted of three layers, which were, from the top to bottom layer, a metal layer with fewer ceramic phases, a ceramic layer with fewer metal phases, and another metal layer with fewer ceramic phases. The ceramic phases were mainly M₃B₂-type borides, and the metal phases were mainly {Fe, Ni} alloys. The appearance of Fe-enriching metal phases was due to the supply of Fe elements from Q235 substrates. With squash pretreatment and without a remelting aftertreatment, ceramics uniformly dispersed in the cermet coatings, and their sizes decreased. The results of microhardness showed that the microhardness of the coating first increased and then decreased from the top layer to the bottom layer, and maximum microhardness was obtained in the layer of ceramics with less metal phases. An electrochemical corrosion test showed that the cermet coatings (<i>j</i>_corr = 6.35 &#956;A/cm²) could improve the corrosion resistance of Q235 steels (<i>j</i> = 43.76 &#956;A/cm²) by one order of magnitude.